The integrator approach is one of the most popular approaches in the field of high-resistance measurement. When applied to the measurement of high-resistance, this approach avoids several innate drawbacks of high-resistance devices.
The integrator approach employs the principle of an analog integrator by recording the time during which the level of the output terminal of the integrator changes to another level as the "integrating time" and using the integrating time as a basis to give the value of a device having a high-resistor. The resistance value is then displayed on the screen of the teraohmmeter.
FIG. 1 is illustrates a block diagram of a conventional teraohmmeter. In this figure, 1 represents a DC voltage source which supplies DC voltage to the resistor, 2 is a circuit for providing reference voltages and a test voltage, 3 is an operational amplifier, 4 is a level comparator which compares the test and reference voltages or levels, 5 is a counter, 6 is a standard frequency generator which generates a standard frequency, and 7 is a gate. R.sub.X represents the test resistor which is a device with high resistance to be measured. The voltage as provided by the DC voltage 1 is divided by circuit 2 into a reference voltages V.sub.1 and V.sub.2 and a test voltage V.sub.T at the ratio 2. The test voltage V.sub.T is transmitted through the test resistor R.sub.X and integrated at the operational amplifier 3, and reaches the level comparator 4. Relay RL.sub.1 is used to start/stop the integrating operation. C is an integrating capacitor.
Under normal conditions, the relay RL.sub.1 is closed and the capacitor C is precharged. During measurement, the relay RL.sub.1 is open and the integrating process is started. The level comparator 3 compares the voltage V.sub.0 and the counter 5 counts the integrating time dt of the voltage V.sub.0, as the reference it changes from V.sub.1 to V.sub.2, according to the signals generated by the standard frequency generator 6. The integrating time is then provided to a calculation device (not shown). The calculation device calculates the resistance value of the resistor R.sub.X according to the equation as described hereinafter.
The calculation of the resistance R.sub.X includes the following equation:
Given that R.sub.X represents the resistance value of the resistor, i.sub.R represents the current of the output terminal of the resistor R.sub.X, V.sub.T is the voltage of the input terminal of the resistor R.sub.X, the capacitance of the capacitor C is C, the voltage difference of the operational amplifier 3 is dV.sub.0 and the integrating time is dt, then:
According to the principle of the operational amplifier: ##EQU1##
The value of R.sub.X may be given from the value of C, dt and the voltage ratio V.sub.T /dV.sub.0.
In the conventional teraohmmeter as described above, the resistance value R.sub.X is a function of the integrating capacitance C. As a result, the accuracy of the measurement of the resistor R.sub.X is dependent on the accuracy and stability of the integrating capacitor C. When a drift happens in the integrating capacitor C, accurate measurement of the resistor R.sub.X may not be achieved.